Composition For Preparing Ceramic Fiber And A Biosoluble Ceramic Fiber Prepared Therefrom For Heat Insulating Material At High Temperature

ABSTRACT

The present invention relates to a composition for preparing ceramic fiber and a biosoluble ceramic fiber prepared therefrom for heat insulating material at high temperature, more specifically, a composition for preparing ceramic fiber comprising SiO 2  as a network-forming oxide, CaO and MgO as modifier oxides, and ZrO 2 , Al 2 O 3  and B 2 O 3  as intermediate oxides with appropriate ratios, which improves the solubility of the ceramic fiber in artificial body fluid; shows good thermal/mechanical properties such as heat resistance, high-temperature viscosity, compressive strength and restoration when used at a high temperature of 1260° C.; and provides an economic effect that a ceramic fiber can be prepared easily by using the existing facilities, and a biosoluble ceramic fiber prepared therefrom for heat insulating material at high temperature.

TECHNICAL FIELD

The present invention relates to a composition for preparing ceramic fiber and a biosoluble ceramic fiber prepared therefrom for heat insulating material at high temperature, more specifically, a composition for preparing ceramic fiber comprising SiO₂ as a network-forming oxide, CaO and MgO as modifier oxides, and ZrO₂, Al₂O₃ and B₂O₃ as intermediate oxides with appropriate ratios, which improves the solubility of the ceramic fiber in artificial body fluid; shows good thermal/mechanical properties such as heat resistance, high-temperature viscosity, compressive strength and restoration when used at a high temperature of 1260° C.; and provides an economic effect that a ceramic fiber can be prepared easily by using the existing facilities, and a biosoluble ceramic fiber prepared therefrom for heat insulating material at high temperature.

BACKGROUND ART

In general, ceramic fibers are used as a raw material of lagging materials, cold-insulating materials, heat-insulating materials, sound-proofing materials, sound-absorbing materials, filtering materials and the like because they have low thermal conductivity, and are thin and long in shape.

The term of “refractory heat-insulating material” usually refers to a refractory fiber that can be used at a temperature higher than that of the conventional mineral wool. In accordance with ASTM C892, fiber-phase blanket heat-insulating materials for the high temperature-application are classified as Type 1 (732° C.) to Type 5 (1649° C.). The safe use temperature of fiber is ordinarily defined as a temperature having a thermal linear shrinkage of 3% or less (or 5% or less) when retaining the fiber at the relevant temperature for 24 hours.

The refractory heat-insulating material which is most generally used now is Al₂O₃—SiO₂(RCF-AS)-based fibers, and the safe-use temperature thereof is in the range of 1100-1260° C. The following literature can be exemplified as prior technologies regarding the Al₂O₃—SiO₂(RCF-AS)-based fibers.

U.S. Pat. Nos. 2,873,197 and 4,555,492 disclose an Al₂O₃—SiO₂(RCF-AS)-based fiber in which a certain amount of ZrO₂ component is added to Al₂O₃—SiO₂-based composition, and said patents states that the safe-use temperature of the fibers disclosed therein has been increased to 1430° C. U.S. Pat. No. 4,055,434 discloses a fiber composition in which at most 16% of burned dolomite, as a raw material of CaO and MgO, is added to Al₂O₃—SiO₂-based composition, said fiber having a heat-resistant temperature of 760-1100° C. U.S. Pat. No. 3,687,850 describes a silica fiber containing 76-90% of SiO₂ and 4-8% of Al₂O₃ as prepared by adding an acid to a fiber composition consisting of SiO₂, Al₂O₃, R₂O (alkali metal oxide), RO (alkali-earth metal oxide) and B₂O₃, and then dissolving RO, R₂O and B₂O₃ therein, said silica fiber having a heat resistance of 1093° C. without the precipitation of any crystalline material.

Although the fiber compositions for the preparation of refractory heat-insulating material has been deduced in light of the heat resistance and the dissolving property to acids, however, they do not pertain to the dissolving property to a salts solution as an artificial body fluid. Furthermore, said fibers can result in a low-solubility problem in a physiological medium because their content of Al₂O₃ exceeds 4%.

According to the recent research materials, it has been reported that, if the crushing pieces of fibers having low solubility in a physiological medium are inhaled and accumulated into the lungs through respiration, it could injure a person's health. Research on the inorganic fiber composition for satisfying the requirements of high-temperature physical property while simultaneously minimizing the possibility of harm to the human body by way of increasing the solubility in a physiological medium is actively proceeding, and examples of the fiber glass composition as developed according to above include the following:

Bioabsorbable Fiber Glass Composition containing CaF₂, ZnO, SrO, Na₂O, K₂O, Li₂O, etc. in addition to CaO and P₂O₅ [U.S. Pat. No. 4,604,097]; Fiber Composition in which P₂O₅ and the like are added to a conventional soda-lime borosilicate [International Publication No. WO 92/0781]; and Fiber Composition which is formed by increasing the amount of P₂O₅ in the soda-lime borosilicate and adding Na₂O and the like thereto [U.S. Pat. No. 5,055,428].

However, said compositions are limited in that it is impossible to use them as a biodegradable material at a high temperature of 1000° C. or higher because: the fibers produced therefrom have low heat resistance since the compositions contain a relatively large amount of R₂O component; they are nothing but an architectural heat-insulating material applicable at a maximum temperature of 350° C. or less in light of having no description of its safe use temperature.

Meanwhile, as a method of fiberizing ceramic fiber composition, a blowing method in which the composition is fiberized by compressed air or compressed steam, and a spinning method in which the composition is fiberized by dropping melt material into a cylinder rotating at a high speed are well known in this technical field. The ideal viscosity of the composition, which is suitable for fiberizing it according to the spinning method or the blowing method, should be low at a range of 20-100 poises, or be similar to that of the conventional SiO₂—Al₂O₃-based composition without any great deviation. If the viscosity of the fiber is too high at the fiberizing temperature, the diameter thereof becomes larger at the same time the amount of the thick unfiberized shot is increased, whereas, if the viscosity of the fiber is too low, the fiber becomes shorter and thinner and the amount of fine unfiberized shot is increased. In general, the viscosity of the molten glass depends on the composition and temperature of the glass. In view of the above, it is necessary to design the optimal composition for retaining the optimal fiberizing viscosity; and the fiberizing composition having the high viscosity has to effectuate it at a higher temperature. Therefore, it is required to stay within a suitable range of viscosity in the vicinity of the fiberizing temperature.

Furthermore, the ceramic fiber which is used for the purpose of heat insulating at a high temperature is required to have high heat resistance as well as excellent endurance against repetitive thermal stress long dash for example, raw material of a furnace. In the ceramic fiber, its use temperature is related to shrinkage at the relevant temperature. Shrinkage of the fiber article is influenced by a viscosity of the glassy fiber composition at a high temperature, a type or amount of the crystal forming and growing due to heat exposure during the life of the article, a crystal precipitation temperature and a viscosity of the glassy material remaining after the crystal is precipitated. Because the high temperature-precipitating crystal has a specific gravity higher than that of the glassy fiber, the stress is caused by the precipitation and growth of the crystal at its interface, and the fiber can be cut or modified by the relulting stress, thereby shrinking it. If the fiber exists in a glassy phase without precipitating any crystal, the viscosity of such a fiber—for example, glass—is gradually lower at a relatively low temperature, and therefore, its shrinkage is increased. Accordingly, the fiber comprised of a composition having low shrinkage at high temperature is required to have the precipitation amount, precipitation velocity and precipitation temperature suitable for precipitating crystal. Also, the variation of the solubility in artificial body fluid must be as small as possible. Therefore, it is important to choose a composition that has low-heat linear shrinkage at high temperature and is easier to melt and fiberize, as well as having high solubility in an artificial body fluid.

In addition, although such materials as glass wool, mineral wool and ceramic fiber have dissolving property in an artificial body fluid better than that of which has been proven to be asbestos, harmful to human body, it has not been yet found whether or not they are harmful to the human body. It is reported that the fiber having a solubility constant of at least 100 ng/cm²·hr does not result in any fibrosis or tumor in an animal inhalation test, even though it became known as having a specific correlation between the solubility of the fiber in an artificial body fluid and the harmfulness in an animal test, in accordance with the toxicological testing result through an animal test [Inhalation Toxicology, 12:26˜280, 2000, Estimating in vitro glass fiber dissolution rate from composition, Walter Eastes]. In the test of biodegradability actually using an artificial body fluid, values of K_(dis) have the maximum error of ±30%, and thus it can be called a biodegradable fiber when the fiber has K_(dis) of at least 150 ng/cm²·hr, more preferably at least 200 ng/cm²·hr.

PRIOR ART PUBLICATIONS Patent Publications

-   U.S. Pat. No. 2,873,197 -   U.S. Pat. No. 4,555,492 -   U.S. Pat. No. 4,055,434 -   U.S. Pat. No. 3,687,850 -   U.S. Pat. No. 4,604,097 -   U.S. Pat. No. 5,055,428 -   International Publication No. WO 92/00781

Non-Patent Publications

-   Inhalation Toxicology, 12:26˜280, 2000, Estimating in vitro glass     fiber dissolution rate from composition, Walter Eastes

CONTENTS OF THE INVENTION Purpose of the Invention

The present invention seeks to solve the problems of the prior arts as explained above. Therefore, the purpose of the present invention is to provide a novel ceramic fiber composition which maintains a high-silica region of SiO₂ content of 75 wt % or more in a CaO—MgO—ZrO₂—SiO₂-based composition system and shows a good fiberization yield, a low thermal conductivity, a low thermal linear shrinkage of 3% or less even at 1260° C. for 24 hours and an excellent biodegradability with a solubility constant in artificial body fluid of 200 ng/cm²·hr or higher.

Constitution of the Invention

To achieve the above purpose, the present invention provides a composition for preparing a biosoluble ceramic fiber for heat insulating material at high temperature, comprising 75-80 wt % of SiO₂, 10-14 wt % of CaO, 4-9 wt % of MgO, 0.1-2 wt % of ZrO₂, 0.5-1.5 wt % of Al₂O₃ and 0.1-1.5 wt % of B₂O₃.

According to a preferred embodiment of the present invention, the sum of the amounts of CaO and Al₂O₃ in the above composition for preparing a ceramic fiber is 11-15 wt %.

Another aspect of the present invention provides a biosoluble ceramic fiber for heat insulating material at high temperature which is prepared from the composition for preparing a ceramic fiber according to the present invention and satisfies one or more of the properties of 1) an unfiberized shot content of 50 wt % or less [e.g., from 0.01 or less to 50 wt %], 2) an average fiber diameter of 6 μm or less [e.g., from 2 to 6 μm], 3) a thermal linear shrinkage (1260° C./for 24 hours) of 3% or less [e.g., from 0.001 or less to 3%] and 4) a solubility constant in artificial body fluid of 200 ng/cm²·hr or more [e.g. from 200 to 1000 ng/cm²·hr or more].

The composition for preparing a ceramic fiber of the present invention decreases the content of Al₂O₃ to a proper level and increases the contents of modifier oxides in a ceramic fiber composition system for heat insulating material at high temperature, thereby remarkably increasing the solubility of the ceramic fiber in artificial body fluid. In addition, the lowering of heat resistance according to the decrease of Al₂O₃ content is overcome by the addition of ZrO₂, a eutectic region which can be generated amid the existence of three components of SiO₂—Al₂O₃—CaO is suppressed by controlling the contents of CaO and Al₂O₃, and the decrease of biodegradability of the ceramic fiber according to the decrease of CaO content is suppressed by the addition of B₂O₃.

The composition for preparing a biodegradable ceramic fiber according to the present invention is explained hereinafter in more detail, according to its constitutional components.

SiO₂ is a main component of ceramic fiber and is contained in an amount of 75-80 wt %, preferably 76-78 wt %, based on the total weight of the composition. If the content of SiO₂ is less than 75 wt %, the contents of CaO and MgO should relatively increase to improve the biodegradability, which results in the problems that the cost for raw material increases, the fiber length becomes too short and thus the stiffness increases, unfiberized shot content increases and thus the fiberization becomes difficult, and the thermal shrinkage increases and thus properties deteriorate. In contrast, if the content of SiO₂ exceeds 80 wt %, there are drawbacks such that the melting of the composition is difficult, and the fiberization viscosity is elevated and thus the diameter of the fiber produced becomes large and at the same time, many thick unfiberized shots are generated.

CaO is a modifier oxide to increase the solubility of the produced fiber in body fluid and contained in an amount of 10-14 wt %, preferably 10-13.7 wt % and more preferably 12-13 wt %, based on the total weight of the composition. If the content of CaO is less than 10 wt %, the solubility of the fiber in body fluid decreases. In contrast, if the content of CaO exceeds 14 wt %, the amount of crystallite precipitated during fiber production increases and thus the SiO₂ content in the produced fiber relatively decreases, thereby causing problems in the thermal stability and the increase of thermal linear shrinkage at high temperature. Furthermore, when the three components of SiO₂—Al₂O₃—CaO exist, a eutectic point can be generated in a eutectic region and the melting may occur at about 1170° C. (see FIG. 1). If the composition part of such a eutectic region exists during the melting procedure of ceramic fiber, the required heat resistance cannot be satisfied and a lowering of heat resistance and heat retention caused by rapid fiber deterioration also occurs. In the present invention, such problems of a eutectic region are solved by controlling the sum of the contents of CaO and Al₂O₃ to 10.5-15.5 wt %, preferably 11-15 wt %.

MgO is another modifier oxide to improve the solubility of the produced fiber in body fluid and contained in an amount of 4-9 wt %, preferably 5-7 wt %, based on the total weight of the composition. If the content of MgO is less than 4 wt %, the biodegradability of the fiber in body fluid decreases or the effect of inhibiting the growth of fiber crystallite during fiber production decreases. In contrast, if the content of MgO exceeds 9 wt %, the eutectic point region becomes close to those of diopside and wollastonite and thus the fiberization viscosity increases and the fiber melting temperature becomes lower. In the present invention, as the component of MgO, raw materials such as dolomite and limestone which are commercially available at relatively low costs may be used instead of a pure compound.

Al₂O₃ is added as an intermediate oxide to perform the function of cutting the bonding structure of SiO₂ during the melting procedure at high temperature and to control the viscosity properly for preparing ceramic fiber. Al₂O₃ is contained in an amount of 0.5-1.5 wt %, preferably 0.7-1.2 wt %, based on the total weight of the composition. If the content of Al₂O₃ is less than 0.5 wt %, the effect of controlling viscosity at high temperature becomes lower. In contrast, if the content of Al₂O₃ exceeds 1.5 wt %, the solubility of the fiber in body fluid decreases and at the same time, the heat-resistant temperature becomes lower.

ZrO₂ is added to prevent the problems of lowering thermal stability at high temperature and chemical durability, which may be caused by the decrease of the content of Al₂O₃, and contained in an amount of 0.1-2 wt %, preferably 0.6-1.5 wt %, based on the total weight of the composition. If the content of ZrO₂ is less than 0.1 wt %, the thermal stability at high temperature and chemical durability become lower. In contrast, if the content of ZrO₂ exceeds 2 wt %, the solubility of fiber in body fluid decreases severely.

B₂O₃ is an oxide forming a glass with a low melting point and added as a fiberization aid to further improve the solubility of the produced fiber in an artificial body fluid. B₂O₃ is contained in an amount of 0.1-1.5 wt %, preferably 0.7-1.3 wt %, based on the total weight of the composition. If the content of B₂O₃ is less than 0.1 wt %, the solubility in an artificial body fluid becomes lower and accordingly the biodegradability in a body fluid decreases. In contrast, if the content of B₂O₃ exceeds 1.5 wt %, the heat resistance becomes deteriorated upon long-term exposure to high temperature and thus the high-temperature shrinkage increases. B₂O₃ is preferably added in case of producing the fiber with an increased SiO₂ content for the following reason: In case of high content of SiO₂, the viscosity of the composition becomes higher according to the increase of the SiO₂ content, thereby lowering the yield of the fiber. However, the addition of B₂O₃ can solve the problems of the lowering of fiber yield and the decrease of solubility in an artificial body fluid. Furthermore, a side effect of fiber solubility decrease in body fluid at room temperature, which may occur when the Al₂O₃ content is increased to control the viscosity at high temperature, can be overcome by the proper use of B₂O₃.

In addition, the composition for preparing a ceramic fiber according to the present invention may contain impurities such as Na₂O, K₂O, TiO₂ and Fe₂O₃, depending on the raw materials used. However, if their content is maintained to a level of 1 wt % or less based on the total weight of the composition, the properties of the produced fiber are not deteriorated by the impurities.

There is no particular limitation to the methods for preparing the composition for preparing a ceramic fiber according to the present invention. Thus, conventional methods for preparing ceramic fiber compositions are available by using the aforementioned components with the aforementioned amount ranges. For example, methods such as electrical melting process may be used but not limited thereto.

Concretely, a melting process of electrically charging type using three-phase electrodes may be employed to prepare the composition for preparing a ceramic fiber. The materials of the electrode and the outlet may consist of molybdenum. By such a melting process, electric-resistant heating is induced, through which a high temperature melting at usually 2000° C. or more is possible.

There is no particular limitation to the methods for fiberizing the composition for preparing a fiber of the present invention. Thus, a conventional fiberization method such as a blowing method or a spinning method may be employed. In employing such a fiberization method, the viscosity range required of the composition for preparing a fiber is preferably 20-100 poise. Viscosity of a melt is a function of temperature and the corresponding composition ratio and thus that of a melt having the same composition ratio is dependent upon temperature. This affects the fiberization since if the temperature of a melt during the fiberization becomes higher, the viscosity decreases whereas if the fiberization temperature becomes lower, the viscosity increases. If the viscosity of the fiber composition at the fiberization temperature is too low, the produced fiber is too short and thin, and many fine unfiberized shots are generated, by which the fiberization yield becomes lower. If the viscosity is too high, fibers having a large diameter are formed and thick unfiberized shots are increased.

The ceramic fiber prepared from the composition for preparing a ceramic fiber of the present invention satisfies one or more of the properties, most preferably all of the properties of 1) an unfiberized shot content of 50 wt % or less, preferably 40 wt % or less, 2) an average fiber diameter of 2 to 6 μm, preferably 3 to 5 μm, 3) a thermal linear shrinkage (1260° C./for 24 hours) of 3% or less, preferably 2% or less, and 4) a solubility constant in an artificial body fluid of 200 ng/cm²·hr or more, preferably 300 ng/cm²·hr or more. Therefore, the ceramic fiber is particularly suitable for heat insulating material at high temperature and shows an excellent biosolubility. Furthermore, it can be produced easily by using conventional ceramic fiber production processes directly and thus provides an economical advantage.

Effect of the Invention

According to the present invention, it is possible to obtain a biosoluble ceramic fiber particularly suitable for heat insulating material at high temperature because it can decrease the harmfulness to the human body since it has a remarkably improved solubility in an artificial body fluid as compared with conventional ceramic fibers and thus is easily removable when inhaled into the lungs of a human, and it has an excellent stability at high temperature and excellent mechanical properties such that the thermal linear shrinkage at 1260° C. for 24 hours is less than 3% while overcoming the property deterioration at high temperature which is a problem of conventional biodegradable ceramic fibers.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a phase equilibrium diagram of the three-component system of SiO₂— Al₂O₃—CaO.

CONCRETE EXPLANATION TO CARRY OUT THE INVENTION

The present invention is explained in more detail by the following examples and comparative examples. However, the scope of the present invention is not limited thereto.

Examples 1 to 5 and Comparative Examples 1 to 5

The compositions for ceramic fiber production were prepared with the ingredients and contents thereof as specified in Table 1 below by a melting process of electrically charging type using three-phase electrodes, and then ceramic fibers were produced from the compositions by a conventional process for producing RCF inorganic fibers. Comparative Example 1 represented a composition for producing a conventional RCF, Comparative Examples 2, 4 and 5 represented compositions for testing, and Comparative Example 3 was a composition for producing a product for 1,100 □.

For the produced ceramic fibers, the average fiber diameter, the unfiberized shot content and the production yield were calculated and determined, and the results are shown in Table 1 below.

TABLE 1 Comp. Comp. Comp. Comp. Comp. Ingredients Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Compositional SiO₂ 76.8 78.4 77.6 75.8 78.8 49.5 76.6 66.3 78.3 77.5 ratio (wt %) CaO 13.7 12.5 11.2 13.1 10.8 0.0 16.7 26.5 12.6 13.1 MgO 6.6 6.7 8.9 8.5 7.6 0.0 4.8 6.5 7.6 6.7 Al₂O₃ 0.5 0.9 1.1 1.5 1.0 49.9 1.9 0.1 0.4 1.6 B₂O₃ 0.2 0.2 0.4 0.5 0.9 0.0 0.0 0.5 0.5 0.5 ZrO₂ 1.7 0.9 0.6 0.3 0.5 0.0 0.0 0.0 0.3 0.3 Impurities¹⁾ 0.5 0.4 0.2 0.3 0.4 0.6 0.0 0.1 0.3 0.3 Total 100. 100 100 100 100 100 100 100 100 100 Average diameter (μm) 3.9 4.7 4.2 4.1 3.8 3.7 3.5 3.8 3.9 3.6 Unfiberized shot content (wt %) 29.0 24.0 32.0 28.0 29.0 30.0 30.0 32.0 33.0 31.0 Production yield (%) 72.0 75.0 73.0 75.0 76.0 80.0 75.0 72.0 68.0 74.0 ¹⁾Impurities: Na₂O + Fe₂O₃ + TiO₂

[Determination and Calculation of Physical Properties]

-   -   Average fiber diameter: The diameters of fibers were repeatedly         measured more than 500 times with an electron microscope at a         high magnification of 1,000× or more, and then the fiber average         diameter was calculated therefrom.     -   Unfiberized shot content: The unfiberized shot content was         determined according to ASTM C892. That is, the ceramic fiber         was thermally treated at 1260° C. for 5 hours and then, about 10         g of specimen was precisely weighed at a degree of 0.0001 g         precision (W₀). The specimen was put into a sieve of 30 mesh         size and pressed with a rubber rod to pass through the 30-mesh         sieve, and then 50-mesh sieve and 70-mesh sieve, subsequently.         The weight (W₁) of particles remaining on each sieve was         measured to calculate the unfiberized shot content (W_(s)) by         using Equation 1 below:

$\begin{matrix} {W_{s} = {\frac{W_{1}}{W_{0}} \times 100}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack \end{matrix}$

wherein, W_(s) represents the unfiberized shot content, W_(o) represents the weight of initial particles and W₁ represents the weight of residue particles.

-   -   Production yield: The production yield was calculated as a ratio         of the total amount of the fiberized material to the total         amount of the drained melt for a given time by using Equation 2         below:

Production yield (%)=[Total amount of fiberized material/hour]/[Total amount of drained melt/hour]  [Equation 2]

In general, if fibers have a large average diameter and a coarse cross-section, they have problems of causing a tingling feel during handling as well as the reduction of the heat-insulating effect. However, the fibers produced according to the examples of the present invention were of high quality because they had a smaller average diameter of 3.8-4.7 μm as compared with 6 μm, which is the average diameter of commercial ceramic fibers as usually produced. Furthermore, because of the smaller average fiber diameter, the fibers produced therefrom are expected to exert a better heat-insulating effect.

Upon comparing the examples of the present invention with the comparative examples, the unfiberized shot contents of the examples of the present invention were 24-32 wt %, which were remarkably reduced values as compared with 30-36 wt % of the comparative examples 1, 2, 4 and 5 of conventional ceramic fibers.

In terms of the production yield, the examples of the present invention achieved production yields of 72-76%, which was equal to or better than those of the conventional ceramic fibers of 70-80%. The comparative example 4, which was a composition having high fiberization viscosity, showed a production yield lower than that of the comparative example 1 and lower than those of the examples.

From the above examples for fiberization, it has been confirmed that according to the present invention, ceramic fibers having an unfiberized shot content of 50 wt % or less and a fiber average diameter of 6 μm or less can be produced by using the existing facilities.

Next, for the ceramic fibers produced in the examples and the comparative examples, the thermal linear shrinkages (1260° C., for 24 hrs/168 hrs), melting temperatures, fiberization temperatures and solubility constants (K_(dis)) in the artificial body fluids were calculated and determined, and the results are shown in Table 2 below.

TABLE 2 Comp. Comp. Comp. Comp. Comp. Time EX. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 1 Ex 2 Ex. 3 Ex. 4 Ex 5 1260° C.  24 hrs 1.6 1.3 1.5 1.9 1.6 1.3 5.9 4.8 5.9 5.3 Linear shrinkage (%) 168 hrs 2.5 2.2 2.8 2.4 2.3 2.1 7.8 6.0 7.8 7.2 Melting temperature (° C.) 1465 1420 1431 1410 1402 1460 1305 1310 1470 1352 Fiberization temperature (° C.) 1843 1861 1847 1830 1831 1835 1829 1814 1902 1841 K_(dis) 345 342 371 381 485 10 132 720 353 227

[Determination and Calculation of Physical Properties]

-   -   Thermal linear shrinkage (variation of fiber length at high         temperature): 220 g of fiber was sufficiently dispersed in an         aqueous 0.2% starch solution and the dispersed fiber was poured         into a 300×200 mm mold, and then said fiber was leveled so as to         have low surface deviation, and the mold was drained through its         bottom to prepare a pad. The obtained pad was sufficiently dried         in an oven at 50° C. for more than 24 hours and cut into a size         of 150×100×25 mm to prepare a test sample. The test sample was         marked with highly refractory materials such as platinum,         ceramic or the like, and the distances between the test marks         were precisely measured by using vernier calipers. The test         sample was then placed in a furnace, heated at 1260° C. for 24         hours and 168 hours, and cooled slowly. For the cooled test         sample, the distances between the test marks were measured to         compare them with those before the thermal treatment. The         thermal linear shrinkages were calculated by using Equation 3         below:

$\begin{matrix} {{{Thermal}\mspace{14mu} {linear}\mspace{14mu} {shrinkage}\mspace{14mu} (\%)} = {\frac{\left( {l_{0} - l_{1}} \right)}{l_{0}} \times 100}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack \end{matrix}$

wherein, l₀ represents the initial distance (mm) between marks on the test sample, and 1₁ represents the length (mm) between marks on the test sample after heating.

-   -   Melting temperature: By utilizing a temperature gradient furnace         capable of controlling the temperature distribution, the         gradient was set within a temperature range of 1100-1500° C. A         ceramic fiber pad was prepared by the same method as that for         measuring thermal linear shrinkage. The prepared pad was cut so         as to have a size of 20 mm×200 mm, and the cut pieces from the         pad were kept within the temperature gradient furnace retained         in a temperature range of 1100-1500° C. for 24 hours. After this         highly thermal treatment, the melt position was observed to         indirectly determine the melting temperature.     -   Fiberization temperature: A blowing method and a spinning method         are used in the fiberization process by melting and fiberizing a         composition for fiber production. The viscosity required of the         composition in such processes is about 20-100 poises. In the         present invention, the fiberization temperature is defined as a         temperature of the composition necessary for retaining a         viscosity of 20 poises. The fiberization temperature varies         according to the composition, and the measurement was based on         the viscosity of the ceramic fiber. Since the melt composition         has a high temperature of 2000° C. or more, the fiberization         viscosity when draining it was indirectly converted. The         fiberization viscosity was calculated as the viscosity ratio to         the theoretical viscosity of the ceramic fiber by using Equation         4 below, from which the fiberization temperature was converted.

$\begin{matrix} {\frac{\eta_{2}}{\eta_{1}} = \frac{F_{1}R_{2}^{2.5}}{F_{2}R_{1}^{2.5}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack \end{matrix}$

wherein, η₁ represents a reference viscosity which is a theoretical viscosity of Al₂O₃—SiO₂ (RCF-AS)-based fiber composition of a conventional ceramic fiber product, η₂ represents a relative viscosity from which the viscosity of the examples and comparative examples were calculated, and the fiberization temperature was converted therefrom, and

-   -   F₁, F₂: Drained melt amount (kg) per hour     -   R₁, R₂: Effective radius (mm) of draining orifice     -   R=Radius (mm) of orifice−tan 15°×{27.99−distance between orifice         and needle} (mm)     -   Solubility constant in an artificial body fluid (Kdis): In order         to evaluate the biosolubility of the produced fiber, the         solubility in an artificial body fluid was determined by the         manner as explained below. In vivo biodegradability of ceramic         fiber was evaluated based on the solubility in an artificial         body fluid. After comparing the retention time in the body on         the basis of said solubility, the solubility constant (K_(dis))         was calculated by using Equation 5 below:

$\begin{matrix} {K_{dis} = \frac{d_{0}\rho \sqrt{1 - {M/M_{0}}}}{2t}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack \end{matrix}$

wherein, d_(o) means an initial average diameter of fiber (μm), ρ represents an initial density of fiber (g/cm³), M₀ represents an initial mass of fiber (mg), M represents a mass of residual fiber after dissolution (mg), and t represents time for test (hr).

A fiber to be tested was placed between thin layers of 0.2 μm polycarbonate membrane filters fixed with a plastic filter supporter, and an artificial body fluid was filtered through said filters to determine the dissolution rate. During the experiment, the artificial body fluid was controlled to retain a temperature of 37° C. and a flow rate of 135 mL/day, and its pH was maintained at a range of 7.4±0.1 by using a gas of CO₂/N₂ (5/95%).

In order to precisely determine the solubility of fiber for a long time, the fiber was leached for 21 days and the filtered artificial body fluid was analyzed with Inductively Coupled Plasma Spectrometry (ICP) to measure the amount of ions dissolved therein at given timings (1^(st), 4^(th), 7^(th), 11^(th), 14^(th) and 21^(st) days). From the results, the solubility constants (K_(dis)) thereof were determined by using the above Equation 5.

The artificial body fluid (Gamble Solution) used for measuring the dissolution rate of the fiber had the contents of ingredients (g/L) as shown in Table 3 below:

TABLE 3 Ingredients Content (g/L) NaCl 7.120 MgCl₂•6H₂O 0.212 CaCl₂•2H₂O 0.029 Na₂SO₄ 0.079 Na₂HPO₄ 0.148 NaHCO₃ 1.950 Sodium tartrate•2H₂O 0.180 Sodium citrate•2H₂O 0.152 90% Lactic acid 0.156 Glycine 0.118 Sodium pyruvate 0.172

After the thermal treatment at 1260° C. for 24 hours, the ceramic fibers of the examples 1-5 showed low thermal linear shrinkage less than 3.0% (1.3-1.9%) and even after the thermal treatment at the same temperature for 168 hours. Furthermore, even if heat-treating the fibers at the above temperature for 168 hours, they still showed low thermal linear shrinkage less than 3.0% (2.2-2.8%). On the contrary, the fibers of the comparative examples 2 to 5 showed rapid shrinkage. Furthermore, in the comparative examples 2, 3 and 5, the melting temperatures were reduced about 50-100° C. or more, as compared with those of the examples.

This can be interpreted as the result of reduced heat resistance due to the presence of a eutectic point in the SiO₂—Al₂O₃—CaO system as explained above. Although the very homogenous mixing is possible in the preparation of ceramic fiber, a local non-homogenous compositional region may exist, and the possibility of existence of a eutectic region in the ceramic fiber increases depending on the existing amount of SiO₂, Al₂O₃ and CaO. Accordingly, the control of CaO and Al₂O₃ contents in the ingredients for the ceramic fiber is an important factor. From the above examples, it has found that the phenomena of heat-resistance reduction due to the existence of a eutectic point can be minimized when CaO content is controlled according to the present invention so as to be within 10-14 wt % and Al₂O₃ content is controlled to be within 0.5-1.5 wt % [i.e., the sum of CaO and Al₂O₃ contents is controlled to be within 10.5-15.5 wt %].

With regard to the fiberization temperature, all of the examples 1 to 5 showed equal levels to 1835° C. of the comparative example 1 which was a conventional ceramic fiber. It is thought that these results were obtained because proper use of Al₂O₃ induced the dissolution of SiO₂ network structure and thus the possible increase of viscosity due to the increase of SiO₂ content was prevented. In the comparative example 4, the fiberization temperature was increased as compared with the examples. In this case, to retain the same viscosity, the melting temperature should be increased considerably. If not, the production yield was reduced as shown in the comparative example 4.

With regard to the solubility constant as a reference for the biodegradability, all of the examples 1 to 5 showed a level of 300 ng/cm²·hr or more and thus are regarded as having excellent biodegradability. Also, the example 5 showed the highest solubility constant (480 ng/cm²·hr) because it contained a relatively large amount of B₂O₃ which is well known to highly contribute to biodegradability. On the contrary, the comparative example 1 as a conventional RCF showed a very low solubility constant. In the comparative examples 2 and 5, it is thought that the relatively high level of Al₂O₃ content of 1.6-1.9 wt % was a cause of the biodegradability reduction. It is also thought that the increase of Al₂O₃ content disturbed the ion exchange and dissolution of the network structure in the mechanism of ceramic fiber dissolution, and thus the solubility of the ceramic fiber was decreased. During the melting procedure at high temperature, Al₂O₃ performs a role of flux material to cut the bonds of SiO₂. However, in the fiberized product obtained through the melting/fiberization procedures, the bonds of Al₂O₃ and the bonds of SiO₂ included in the ceramic composition are reinforced to show the effect of suppressing the dissolution mechanism in a body fluid. The comparative example 3 showed the highest solubility constant. However, since it had a low level of SiO₂ content and a high level of CaO content, the maximum use temperature was restricted to 1100° C. and the shrinkage at 1260° C. was higher than the acceptable limit.

As explained above through the examples and the comparative examples, the ceramic fiber according to the present invention has excellent biodegradability in an artificial body fluid, excellent fiberization property and high productivity due to its high fiberization yield. Furthermore, the ceramic fiber according to the present invention is useful as a heat insulating material at high temperature because the variation of thermal linear shrinkage is small in spite of thermal treatment at 1260° C. for 24 hours. 

1. A composition for preparing a biosoluble ceramic fiber for heat insulating material at high temperature, comprising 75-80 wt % of SiO₂, 10-14 wt % of CaO, 4-9 wt % of MgO, 0.1-2 wt % of ZrO₂, 0.5-1.5 wt % of Al₂O₃ and 0.1-1.5 wt % of B₂O₃.
 2. The composition for preparing a biosoluble ceramic fiber according to claim 1 wherein the sum of the amounts of CaO and Al₂O₃ is 11-15 wt %.
 3. The composition for preparing a biosoluble ceramic fiber according to claim 1 wherein the amount of CaO is 10-13.7 wt %.
 4. The composition for preparing a biosoluble ceramic fiber according to claim 1 wherein the amount of CaO is 12-13 wt %.
 5. A biosoluble ceramic fiber for heat insulating material at high temperature which is prepared from the composition for preparing a ceramic fiber according to any one of claims 1 to 4 and satisfies one or more of the following properties 1) to 4): 1) an unfiberized shot content of 50 wt % or less, 2) an average fiber diameter of 6 μm or less, 3) a thermal linear shrinkage (1260° C./for 24 hours) of 3% or less and 4) a solubility constant in an artificial body fluid of 200 ng/cm²·hr or more.
 6. The biosoluble ceramic fiber according to claim 5 which satisfies all of said properties 1) to 4).
 7. The biosoluble ceramic fiber according to claim 5 which is fiberized by a blowing method or a spinning method.
 8. A heat insulating material comprising the biosoluble ceramic fiber according to claim
 5. 